This is not fair on your part. On one hand premarital relationship is absolutely normal for you and on other hand you are not able to trust her because of her relationships. You should not forget that her partners were married and not she. For her they were premarital affairs and not extramarital. I think it is not pre or extra marital is actually bothering you, it is your male psyche (male can do it but how can female do it?) and probably insecurity that she can cheat you later is scaring you. Discuss your ideas about marriage with her. If she shows honesty in accepting it then you do not have any morally valid point to step back.